That does look like a light leak in the first picture. The second one is drying out and becoming impervious to the developer. I don't know why the third isn't as well exposed as the second.

Your development is very uneven. Does it behave as though it is stuck on the plate and won't cover some areas? Try adding about 10% additional alcohol to the developer to have it flow more evenly.

Unless the light leak shows up again in the same form, same place, I wouldn't worry about it. It might have something to do with how the darkslide was removed or put into the holder at the camera. A slight movement to separate the holder from the camera back when manipulating the darkslide may have caused the streak.

Your background is coming through the image and complicating the view and problem solving. Put those plates against a sheet of black cardboard or a piece of black velvet to make the plate easier to see as displayed normally.